This invention relates to a method and apparatus for the manufacture of glass articles and more particularly to control means for automatically maintaining a relatively constant parison temperature under conditions which cause the flow of cooling air to be at a limit.
One of the most common methods for manufacturing glass articles such as bottles, jars and similar containers is by means of blowing machines which take successive gobs of glass as they are sheared from the outlet of a glass feeder. Those gobs are first fed to blank molds where the parison is formed, then after the parison is formed the individual blanks or parisons are automatically transferred to blow molds where they are blown to final form. At each of the positions in the bottle production process, that is, both where the parison is formed and where the bottle is blown, there are orifices positioned on both sides of the molds so that air is blown onto both sides of the molds thus cooling the molds and the glass in the molds. It is well known that the temperature in the glass parison at the time of blowing affects the distribution of glass in the blowing of the article. It is therefore advantageous to maintain as constant as possible the temperature distribution of the parison as by maintaining the temperature at a particular point on the parison in the production of the successive parisons so that optimum blowing occurs. This temperature may be referred to simply as the parison temperature, it being understood that such a reference relates to the temperature at a fixed point on the parison at a particular time in each successive formation of a parison.
It will be understood that the air cooling system supplying the air to the orifices for the cooling of the parison is normally common for a number of sections of a glass forming machine and sometimes common to several machines. Thus, the main duct which supplies the air branches off to the several sections of each machine. Certain branches of the cooling system for one machine are at times shut off and at other times all of the branches to one of the machines may be shut off, for example, when that particular machine is shut down. In either case the available number of openings allowing air flow will be varied and this variation can have a wide range, as when a whole machine is shut down. Such variations in the number of outlets in the cooling system will naturally cause a variation in the pressure of the system so that the amount of cooling air flowing from the remaining orifices will vary unless the cooling system is subject to pressure control as by the adjustment of the total air flow in the system.
Normally, the air used in cooling is ambient air from outside the plant and therefore the cooling effect of the air is subject to change due to seasonal and daily temperature changes as well as because of the change in the rate of flow resulting from closing some orifices. For example, the temperature of the air may vary over a wide range between winter and summer as well as between day and night, also in some cases variations in the humidity may be significant. Since the temperature can change significantly and since it affects the cooling properties of the air, it is desirable to compensate for change in temperature.
Other factors which influence the temperature of the parison include the average rate of heat input to the molds from the glass being supplied to the molds. A change in the rate of heat input may result, for example, from a change in the mass flow of the glass due to an increased rate of production, that is, an increased number of gobs per minute, or an increased weight per gob of glass, or from an increase in the temperature of the gob as it is supplied to the blank mold. It is necessary when there is a variation of the heat input to the molds to modify the flow of cooling air in order to maintain a constant parison temperature.
Both the changes in the cooling effect of the air and the changes in the average rate of heat input may be considered to be disturbances to the heat transfer equilibrium between the cooling air and the parisons which is maintaining the parison temperature. After such a disturbance it is, of course, desirable that a new equilibrium condition be established to provide the desired parison temperature. The modified equilibrium condition is in effect a new equilibrium level in that the rate of heat transfer is modified, that is the heat input is changed and, of course, the cooling rate must be changed to maintain an equilibrium between them so the parison temperature remains constant.
Prior art systems have attempted to maintain the temperature of the parison constant by measuring the temperature of the parison as closely as possible and by varying the air flow in response to changes in the parison temperature so as to tend to maintain it constant. Thus, the prior art systems have relied upon direct measurements and feedback control as a basis for controlling the temperature. Such direct measurements are difficult to make with any degree of accuracy under conditions such as exist in glass forming machines. Therefore, it is desirable to control the temperature of the parisons without using direct temperature measurement. This is accomplished by the temperature control system disclosed in my U.S. Pat. No. 3,860,407, which issued Jan. 14, 1975. In using the control system described by the patent it is sometimes found that the ambient temperature of the cooling air is either too high or too low for proper operation of the control system. For example, when high summer air temperatures are experienced the cooling effect of the air blown on the molds for the parisons is insufficient for the particular temperature at which the glass feeder is supplying glass to the forming machine for the particular speed at which the forming machine is being operated. Under such conditions the control system of the above mentioned patent will operate to increase the pressure of the cooling air in the ducts to be maximum that can be provided by the fans supplying the air. A similar problem arises when the temperature of the ambient air is low as in the winter. Under such conditions the cooling effect is so great that an unacceptably low flow of air on the molds would result and the cooling would not be sufficiently uniform to maintain the needed product quality. Usually a lower limit for the duct pressure is established to maintain the minimum flow of cooling air.
Under both conditions mentioned above its is necessary to take some action which will maintain the parison temperature as nearly constant as possible. It is therefore an object of this invention to provide an improvement on the control system of the above mentioned patent which will be effective to maintain the parison temperature at its desired value when the ambient temperature conditions are such that the pressure control cannot it the needed range of cooling air flow because of the limits established on the duct pressure.